Why Evolution is True is a blog written by Jerry Coyne, centered on evolution and biology but also dealing with diverse topics like politics, culture, and cats.
This morning Matthew sent me a tweet by “The Dialectical Biologist” (TDB), which astounded me. I don’t know who TDB is, but he/she identifies as “Biologist. Anti-hereditarian. Lewontin fan.” The anti-hereditarian bit explains some of the criticality in the tweet below, and it’s worth noting that Lewontin himself gave The Selfish Gene a very critical review in Nature in 1977 (free with the legal Unpaywall app).
Here’s the tweet (the second part is the important claim), and two of the six subsequent tweets explaining why TDB sees The Selfish Gene as “the most damaging popular science book of all time.”
Regarding "genes = units of selection": I subscribe to the old school view that, at least when discussing organisms, genes should not be considered units of selection. https://t.co/yff9go7V2s
I was, of course, Dick Lewontin’s Ph.D. student, and I loved and admired the man. But I have to add that his Marxist politics, which included views of an almost infinite malleability of human behavior, did affect his science, and I think his review of Dawkins’s book is marred by that ideology. If you read Dick’s review, you’ll see that, like TDB above, Lewontin objects to the lack of discussion of genetic drift, and to Dawkins’s supposed claim (one that he didn’t actually make) that every aspect of every organism was installed by natural selection, accompanied by untestable “adaptive stories” about how it arose. (Lewontin calls this “vulgar Darwinism”.)
In short, Lewontin’s review was an abridged version of his paper with Steve Gould, “The Spandrels of San Marco and the Panglossian paradigm: A critique of the adaptationist program.” That paper was valuable in correcting the excesses of hyperselectionism, pointing out other reasons besides selection for the appearance of organismal traits and behaviors, and implicitly demanding data instead of fanciful stories for natural-selection explanations. (There are many traits, however, like extreme mimicry, where there is no plausible explanation beyond natural selection on bits of DNA.)
It is misguided to fault Dawkins’s book for not dealing in extenso with genetic drift or the San Marco alternatives. The Selfish Gene is essentially a book about how natural selection really works. It’s not important that it doesn’t define “gene” in the way that TDB wants; in fact, biologists haven’t yet settled on a definition of gene! It’s sufficient, when regarding the phenomenon of natural selection, to define a gene as “a bit of DNA that affects the properties of an organism”. If those properties enhance the reproduction of the carrier (the “vehicle”), then the gene gets overrepresented in the next generation compared to the alternative gene forms (“alleles”). These selected bits of DNA act as if they were selfish, “wanting” to dominate the gene pool. That is a very good metaphor, but one that has been widely misunderstood by people who should be thinking more clearly.
The value in the book lies in its clear explanation of how natural selection acts largely (but not entirely) at the level of the gene, not the organism, the group, the population, or the species; its distinction between “replicators” (bits of DNA subject to natural selection) and “vehicles” (the carriers of replicators whose reproductive output can be affected by those replicators); that “kin selection” is, in essence, nothing really different from natural selection acting on the genes of an individual; and that, contrary to a naive “selfish gene” view, altruism can result from natural selection. Finally, it explains clearly the thesis (earlier adumbrated by G. C. Williams) that “group selection—selection on populations—is not a major source of adaptation in nature. (See Steve Pinker’s wonderful essay on the inefficacy of group selection published ten years ago in Edge.)
The Selfish Gene is the clearest explanation I know of how natural selection works, as well as an exposition of ideas like kin selection that were fairly new at the time of the book’s publication. It also introduces the idea of “memes”, which I think is a distraction that has led almost nowhere in the understanding of culture, but that is just a throwaway notion at the end of the book. (You can see my critique of the meme framework in a review of Susan Blackmore’s book The Meme Machine that I wrote for Nature; access is free.)
Think of the book as an explanation for the layperson about how natural selection really works, and you’ll recognize its value. As far as “damaging” the popular understanding of science, that is a grossly misguided accusation. By explicating how natural selection really works, explaining some of its variants (like kin selection), and dismissing widespread but largely erroneous ideas about selection on groups, The Selfish Gene did the public an enormous service. While popularity is not always an index of a science book’s quality, in this case it is: many laypeople have written about how they finally understood natural selection after reading it.
I could, in fact, argue that the San Marco paper by Gould and Lewontin was damaging, too, by overly restricting the domain of natural selection and failing to adduce cases where drift or pleiotropy were not sufficient explanations for traits (mimicry is one), so that natural selection was the most parsimonious explanation. (In the latter part of his career, it was hard to get Steve Gould to even admit that selection was important, much less ubiquitous). But “San Marco” was itself valuable in dampening hyper-Darwinism, and in the main was a good contribution to evolutionary biology. The Selfish Gene was, however, a much better contribution
I asked Matthew, someone who of course knows the ins and outs of evolutionary genetics, if he agreed with TDB’s negative assessment of The Selfish Gene. His reply:
Given I am giving a lecture tomorrow in which I tell 600 students they should all read it, I think not…
When I asked permission to reproduce his quote above, he said “sure” and also me the slide he’s showing his 600 students:
. . .and added this:
FWIW I also show them three views in the levels/units of selection debate (a philosopher who says it has to be genes as they are the only things that are passed down, Dick who says we can’t really know and Hamilton who says it’s complicated and it depends what you look at).
The next section of the lecture deals with social behaviour (hence the final line)
I invite those readers who have read The Selfish Gene to weigh in below with their opinion.
Of the several independent assertions that constitute Darwin’s “theory of evolution” in On The Origin of Species, Darwin regarded the idea of natural selection as his most important and original. After all, it alone explained how naturalistic processes could lead to the remarkable adaptations of animals and plants heretofore seen as some of the strongest evidence for God. And although the idea of evolution itself had been broached by others before Darwin, including his own grandfather Erasmus, natural selection seemed to be sui generis.
Well, not entirely. It was anticipated by several people, including the Scottish polymath James Hutton in 1794. But the most remarkable precursor to the idea of natural selection was published by Scottish horticulturalist and agriculturalist Patrick Matthew (1790-1874) as an appendix to his book On Naval Timber and Arboriculture (1831). Although the book was about how to build ships using wood, and what kind of wood to use, Matthew added a 28-page Appendix. In that Appendix were 29 sentences that laid out what he called “selection by the law of nature”, which bore a striking similarity to the idea made famous by Darwin 28 years later.
You can read Matthew’s appendix at Darwin Online, and some of the statements about his views of natural selection can be seen and discussed in context here,here, and here. ‘
Here are two excerpts from Matthew’s Appendix:
THERE is a law universal in nature, tending to render every reproductive being the best possibly suited to its condition that its kind, or that organized matter, is susceptible of, which appears intended to model the physical and mental or instinctive powers, to their highest perfection, and to continue them so. This law sustains the lion in his strength, the hare in her swiftness, and the fox in his wiles. As Nature, in all her modifications of life, has a power of increase far beyond what is needed to supply the place of what falls by Time’s decay, those individuals who possess not the requisite strength, swiftness, hardihood, or cunning, fall prematurely without reproducing—either a prey to their natural devourers, or sinking under disease, generally induced by want of nourishment, their place being occupied by the more perfect of their own kind, who are pressing on the means of subsistence.
. . . There is more beauty and unity of design in this continual balancing of life to circumstance, and greater conformity to those dispositions of nature which are manifest to us, than in total destruction and new creation. It is improbable that much of this diversification is owing to commixture of species nearly allied, all change by this appears very limited, and confined within the bounds of what is called Species; the progeny of the same parents, under great difference of circumstance, might, in several generations, even become distinct species, incapable of co-reproduction.
The self-regulating adaptive disposition of organized life may, in part, be traced to the extreme fecundity of Nature, who, as before stated, has, in all the varieties of her offspring, a prolific power much beyond (in many cases a thousandfold) what is necessary to fill up the vacancies caused by senile decay. As the field of existence is limited and pre-occupied, it is only the hardier, more robust, better suited to circumstance individuals, who are able to struggle forward to maturity, these inhabiting only the situations to which they have superior adaptation and greater power of occupancy than any other kind; the weaker, less circumstance-suited, being permaturely destroyed. This principle is in constant action, it regulates the colour, the figure, the capacities, and instincts; those individuals of each species, whose colour and covering are best suited to concealment or protection from enemies, or defence from vicissitude and inclemencies of climate, whose figure is best accommodated to health, strength, defence, and support; whose capacities and instincts can best regulate the physical energies to self-advantage according to circumstances—in such immense waste of primary and youthful life, those only come forward to maturity from the strict ordeal by which Nature tests their adaptation to her standard of perfection and fitness to continue their kind by reproduction.
Well yes, that has variation, differential survival, culling of most individuals in a species, speciation, and adaptation—all features of Darwin’s own theory. It’s a remarkable anticipation of Darwin’s ideas.
Does this mean that Matthew deserves credit for the idea of natural selection? Only as an anticipation of Darwin’s far more thorough explication (Darwin, by the way, never read Matthews’ Appendix). Matthew deserves no more credit for natural selection as a popular idea than does Erasmus Darwin for evolution. Matthew’s ideas weren’t adopted, were almost never cited, had no influence in biology, and Matthew never realized until after The Origin was published (and sold out the printing in a single day) that he once had within his grasp The Big Idea that explained the design-like features of nature.
Nevertheless, several people have tried to diminish Darwin’s idea by pointing out that Matthew had it first—and that Darwin plagiarized it. The latest attempt is by Mike Sutton in this book published two months ago (click on image to go to Amazon link):
I haven’t read it, but according to Geoff Cole, a cognitive scientist at the Centre for Brain Science at the University of Essex, who reviewed the book in the latest issue of Evolution (click below for free access), Sutton’s book is a real hit job on Darwin.
The title of Sutton’s book clearly asserts that Darwin took credit for Matthew’s theory, and it’s true that once Patrick Matthew had read The Origin, he argued for his own precedence, even though Darwin had never seen the “incriminating” sentences above. Sutton also claims that Matthew’s idea had real priority because Naval Timber was cited by others before 1859, but as Cole notes in a very critical but polite review, those citations were almost all to the book itself, not to the ideas in the Appendix.
Cole also notes Sutton’s ridiculous accusations of Darwin’s “plagiarism”:
What is most uncomfortable about Sutton’s thesis is his treatment and personal attack on Darwin. He suggests that Darwin ”was a plagiarist who lied repeatedly” and undertook “deliberate, knowing fraud”. Indeed, “the biggest science fraud in history”; fraud that Darwin supposedly hoped “nobody would notice”. Sutton also expresses suspicion about the chronic illness Darwin was known to suffer; a subject that many historians have written about (e.g., Hayman, 2009). From every single account of Darwin and how he went about his life, these “lies” are the complete opposite of what we know about the man. I have lost count of the number of times I have seen a scholar write that a particular event “is testament to his honesty”. As Browne (1985) stated, “By the time Descent of Man was published in 1871 reviewers were falling over themselves to congratulate Darwin’s “unassailable integrity and candour, and his “wonderful thoroughness and truthfulness” (Browne, 1985, p.257 & 258).
Every serious historian who’s studied Darwin’s life knows that he was neither a plagiarist nor a liar, although he did, understandably, want to preserve credit for his own ideas. After Matthew wrote a claim of his priority in The Gardner’s Chronicle in 1859, Darwin not only published an acknowledgement of Matthew’s precedence in the same magazine, but also inserted this long acknowledgment of Matthew’s work into the 3rd edition of On the Origin of Species:
In 1831 Mr. Patrick Matthew published his work on ‘Naval Timber and Arboriculture,’ in which he gives precisely the same view on the origin of species as that (presently to be alluded to) propounded by Mr. Wallace and myself in the ‘Linnean Journal,’ and as that enlarged on in the present volume. Unfortunately the view was given by Mr. Matthew very briefly in scattered passages in an Appendix to a work on a different subject, so that it remained unnoticed until Mr. Matthew himself drew attention to it in the ‘Gardener’s Chronicle,’ on April 7th, 1860. The differences of Mr. Matthew’s view from mine are not of much importance: he seems to consider that the world was nearly depopulated at successive periods, and then re-stocked; and he gives, as an alternative, that new forms may be generated “without the presence of any mould or germ of former aggregates.” I am not sure that I understand some passages; but it seems that he attributes much influence to the direct action of the conditions of life. He clearly saw, however, the full force of the principle of natural selection. In answer to a letter of mine (published in Gard. Chron., April 13th), fully acknowledging that Mr. Matthew had anticipated me, he with generous candour wrote a letter (Gard. Chron. May 12th) containing the following passage:—”To me the conception of this law of Nature came intuitively as a self-evident fact, almost without an effort of concentrated thought. Mr. Darwin here seems to have more merit in the discovery than I have had; to me it did not appear a discovery. He seems to have worked it out by inductive reason, slowly and with due caution to have made his way synthetically from fact to fact onwards; while with me it was by a general glance at the scheme of Nature that I estimated this select production of species as an à priori recognisable fact—an axiom requiring only to be pointed out to be admitted by unprejudiced minds of sufficient grasp.”
Cole explains patiently why Darwin should get nearly all the credit for the idea of natural selection. A few excerpts from Cole’s excellent review:
Who then should be credited with discovering the process by which evolution occurs? Matthew, Hutton, Maupertuis, Wells? Or anyone else who also chipped in? The answer is simple. Charles Darwin.
. . . A necessary condition of insight is that the knowledge must be reflected upon and placed within the appropriate context. Unless a person fully recognises what they have said, done, or found, no formal insight has occurred. There is no priority.
. . . I suspect Matthew was annoyed with himself, as I was with myself, for not realising the importance of what he had written. That may have been why he dedicated so much of his later efforts on his priority claim. If he had realised he would surely have submitted an academic paper outlining his theory; a paper that was only about the theory. Given fear of religious establishment, this could have initially been anonymously penned. He may have even published a book on the origin of all life forms and how the development of every single species can be explained. He would have also repeatedly used his phrase “the process of natural selection”, a phrase Sutton places great emphasis on, as opposed to the one time he did so in Naval Timber. As it was, there was no paper or book. There was no in-depth development of ideas about evolution and how it relates to divergence, heredity, the geological record, geographic distribution, classification, morphology, and embryology. No lengthy discussion of how there are problems and “difficulties” with his own theory. There was not 30 years of methodical work in which he used his theory to explain aspects of cross-pollination and movement in plants, not to mention work on human psychology, sexual behaviour, and emotions. There were no lengthy and numerous discussions with colleagues about his theory and when he should go public.
In fact, Sutton acts like a creationist, arguing that generations of evolutionary biologists have realized that Matthew should really get credit for the idea; but we have, because of our mindless adulation of Darwin, kept that quiet:
Essentially, Sutton has to explain why generations of evolutionary biologists and the like have never come to the same conclusion as himself. The usual explanation is that we are all involved in a “cover up” (p. 5) or part of the “Darwin Industry”, as Sutton calls it, in which a “loosely affiliated in-group of scientists, historians of science, other writers, publishers, editors, and journals, share a common goal to protect the perception of Charles Darwin as a genius science hero” (p. 10). But how This article is protected by copyright. All rights reserved. about this for an alternative explanation? Those generations of biologists have independently decided that there is nothing to see here, that Darwin should be honoured with discovering evolution. Furthermore, if a few sentences in which natural selection is referenced warrants priority, as Sutton seems to believe, then why pick out Patrick Matthew? Why not his predecessors, Hutton, Wells, or Maupertuis? In fact, shouldn’t Matthew be accused of plagiarism, having failed to acknowledge the fact that his ”own original child” was described at least 30 years before by various others?
Sutton’s book is his latest, in his decade-long, attempt to undermine Darwin’s priority. As all others before, this one will fail.
Of that there’s no doubt. Matthew’s independent musings about natural selection are a remarkable coincidence, but he didn’t make much of them, didn’t examine them further, and certainly didn’t try to integrate them into a grand theory of organic evolution. But judge for yourself: I hope you’ve read The Origin, so just peruse Matthew’s brief discussion and then ask yourself whether Matthew should get the lion’s share of the credit for the idea of natural selection.
************
One brief correction of Cole’s fine review: on its first page it describes Darwin as being “the ship’s naturalist” on the voyage of the Beagle. That’s a common misconception, for an “official” naturalist—the ship’s surgeon Robert McCormick—had already been designated. Darwin sailed on the Beagle using his own money, and his position was as both a “self funded naturalist” and also the “captain’s companion”. He was taken aboard largely to provide gentlemanly company for Captain FitzRoy, with whom he dined and conversed. Darwin’s researches and collections during the voyage were done on his own volition and enthusiasm.
All of us have noticed that after a period of immersion in water, the skin on both our fingers and toes wrinkles up, but not the skin anywhere else on our body. Here are two photos of the crenulated digits:
b.) Is there any usefulness or “adaptive significance” of the wrinkling? That is, did natural selection favor it because the wrinkles are useful.
The two articles below, the first a new popular summary from the BBC and the second a year-old scientific paper discussing the “adaptive significance” of the wrinkling, suggest answers to both questions.
It turns out that we know the mechanism of wrinkling pretty well, but, despite the assurance of both articles, we still have no idea whether it’s an “adaptive” response to water or merely some epiphenomenon that makes no difference to our well being or reproductive output. That both articles immediately look for an adaptive “reason” why natural selection promoted finger and toe wrinkling is an example of what Steve Gould called “naive pan-selectionism”: assuming that every feature has natural selection behind the evolution of that feature, and favoring the production of that feature—in this case, wrinkling.
Panselectionists often accept pretty scanty evidence as being supportive of their theory, and I think you can see that here.
Click on both screenshots to read the article; the pdf of the scientific article (in PLOS One; reference at bottom) can be downloaded here.
I’ll use facts from both articles, but quotes will be attributed to one or the other.
First, how long does it take to wrinkle up? It depends on the temperature, with 3.5 minutes in warm water to begin wrinkling (40º C or 104° F) and 10 minutes in tepid water (20º C or 68° F). But even in cool water we will wrinkle.
How does it happen? Scientists first thought that it was simple osmosis: the skin cells absorbed ambient water and that made the cells swell up, causing wrinkles. But then they noticed that if the median nerve in the arm is severed, there is no wrinkling. That rules out the osmosis theory as a complete explanation. Osmosis may contribute a bit to the wrinkling, but nerves and blood vessels are also involved. Author Davis of the PLOS ONE paper says this:
Explanations for the wrinkling of the skin include a passive response of the skin to immersion, or an active process that creates the wrinkles for a functional purpose. There is overwhelming evidence that finger-wrinkling is an active process. The small blood vessels of the fingertip constrict, which creates valleys in the skin surface, triggered by water entering sweat pores . Note that a passive explanation would usually assume that water absorbs into the skin, pushing up ridges. This vasoconstriction appears to occur most readily at a temperature of around 40° Celsius, or the temperature of a warm bath [2]. People with autonomic neurological conditions including Parkinson’s, cystic fibrosis, congestive heart failure or diabetic neuropathy may show abnormal or asymmetric wrinkling in the affected parts of the body.
Note that in the first sentence he conflates an “active process” with “an adaptation that has a functional purpose.” This isn’t necessarily true. We get wrinkles, gray hair, and liver spots with age, which are “active processes,” but that doesn’t mean those features are the direct products of natural selection. (What is the adaptive function of liver spots?). The BBC adds a bit more about the mechanism:
Wilder-Smith and his colleagues proposed that when our hands are immersed in water, the sweat ducts in our fingers open up to allow water in, which leads to an imbalance in the salts in our skin. This change in the salt balance triggers the firing of nerve fibres in the fingers, leading to the blood vessels around the sweat ducts to constrict. This in turn causes a loss of volume in the fleshy area of the fingertip, which pulls the overlying skin downwards so that it distorts into wrinkles. The pattern of the wrinkles depends on the way the outermost layer of skin – the epidermis – is anchored to the layers beneath it.
The involvement of nerves explains why some conditions that affect nerves (see first indented para above) affect skin wrinkling.
Let’s assume, then, that we have a pretty good idea of how this happens in fingers, though nobody says much about toes or the rest of the body. (Toes are also sorely neglected in the “adaptive” explanation.
Both sets of authors then set about explaining why natural selection would favor such wrinkling (again, they discuss only fingers, not toes). The experiment describe in the second link above, which gives results in line with previous studies, suggests that the wrinkled skin allows you to grab wet objects with more force than if your skin is unwrinkled and wet. And if your fingers are wrinkled, you’re likely to be in an environment where there are wet objects. The purported mechanism for this is the same one for treads and valleys in tires: the “channels” in our finger wrinkles suposedly help squeeze out the water when we’re gripping a wet object, allowing better contact with the object. (But what about the toes?)
Davis, then, did a study estimating the strength it took to grip a small and initially DRY plastic disk under three conditions:
a. dry unwrinkled fingers
b. wet wrinkled fingers (note: they apparently didn’t use dry wrinkled fingers, but it’s not clear from the paper. In fact, if they used dry unwrinkled fingers, it would make the adaptive explanation less credible.)
c. wet unwrinkled fingers
Not did they use wet objects, which is crucial for their adaptive hypothesis, though of course gripping a plastic disk with wet wrinkled fingers will make the object wet. Note also that the object is small and light (the BBC says it weighed as much as a couple of coins).
I won’t go into the detail to measure force, but they had an apparatus that measured both grip strength and the ability of the subject to lift up the object and hold it sufficiently tightly so it could be manipulated to follow a computer track. Here’s a photo from the paper:
(From paper): Fig 1. Picture of the equipment in use. The participant is gripping a load cell between finger and thumb. The participant’s task is to pull up on the second load cell to match a force trace that appears on the laptop monitor. The current load force is shown as a red circle, and the history of the participant’s force is shown as a trail of green dots.
The results: people with wet wrinkled fingers and those with dry fingers had similar grip forces, but those with wet, unwrinkled fingers needed significantly more force to grip the disk. Here’s one graph (just look at the top three lines) showing no significant difference between wrinkled-finger force (red) and dry-fingered force (purple), but significantly more force needed using wet, unwrinkled fingers. (The paper give statistics). This shows no real benefit of wet, wrinkled fingers over dry fingers when gripping the disk, but if your fingers are wet and unwrinkled, it’s harder to grip (the plastic get slippery).
(From paper): Fig 2. Comparison of performance across conditions. Mean grip force (thinner traces) and load force (thicker traces) when participants tracked a load weight target (black line). Participants with wrinkled fingers produced a grip force that did not differ from that used by people with dry fingers in the static hold phase, however people with wet but non-wrinkly fingers used a significantly higher amount of grip. The shaded area indicates the pointwise ±1 standard error of each mean trace. Lines below the trace indicate the attack phase (A) of the trial, the static phase (S) and the decay phase (D).
Here’s another graph that shows pretty much the same thing, but showing the grip force needed to sustain the load of the plastic disk under the same three conditions but with varying “load force” (weight, which could be manipulated). Green is wet, unwrinkled fingers, red is wet, wrinkled fingers, and blue is dry unwrinkled (normal) fingers:
(From paper): Fig 4. Relationship between grip and load force in Dry, Wet and Wrinkly conditions. This illustrates the grand mean of the grip and load forces for the whole duration of the trail, minus the first 1000 ms. The target force is shown as a dashed line. The three grip force traces lie above this line, indicating the safety margin. The ‘easiest’ condition, Dry (blue trace) follows the target force most closely. The ‘hardest’, Wet (green trace), shows a higher safety margin, and looser coordination. Participants with Wrinkly fingers (red trace) lie between the two.
Wet unwrinkled fingers require more force to hold the disk than do dry ones. Wet, wrinkled fingers aren’t superior to either, but intermediate between them. (No statistics are given, but another graph implies that none of the differences between the lines in the plot right above are significant.)
The overall conclusion is not strong. Clearly, wet unwrinkled fingers make it harder to grip a smooth plastic object than either dry fingers or wet wrinkled fingers (DUH), but wet wrinkled fingers don’t make it easier to grasp an object than dry unwrinkled fingers. In other words, any advantage of wrinkling is only when it’s compared to wet unwrinkled fingers. Otherwise, dry fingers grasping a dry object are marginally (and nonsignificantly) better than wet, wrinkled fingers.
What can you conclude from this? I’d say, “not much”, but the author of both the BBC article and of the paper seem to think that wrinkling is an adaptation that evolved in our ancestors to enable them to grip objects under wet conditions:
BBC:
This suggests that humans may have evolved fingertip and toe wrinkling at some point in our past to help us grip wet objects and surfaces.
“Since it seems to give better grip under water, I would assume that it has to do with either locomotion in very wet conditions or potentially with manipulating objects under water,” says Tom Smulders, an evolutionary neuroscientist at Newcastle University who led the 2013 study. It could have given our ancestors a key advantage when it came to walking over wet rocks or gripping branches, for example. Alternatively, it could have helped us when catching or foraging for food such as shellfish.
From the paper:
Grip and load force coordination is an important aspect of object handling. The ability to generate the correct amount of grip force for a given load means that the minimum necessary amount of energy is used by the muscles that control the fingers and hands, and means that objects are less likely to be dropped or to be crushed. Efficient grip force coordination is seen in many extant primates, and is likely to have evolved early in the primate lineage [13]. The grip force required to stabilise a wet object is greater than the force required for a dry object, since the coefficient of friction of the digit-object interface is reduced [8]. It would therefore benefit an animal to gain an advantage in handling wet objects, as this would increase success in hunting and foraging in watery environments. The skin of the fingertip is already adapted for regulation of moisture at the contact surface [14]. Fingertip wrinkles would seem to afford an enhanced advantage in object handling, and may plausibly aid travel and clambering in wet areas, especially if combined with wrinkled toes.
Ergo, it helped us “hunt and forage in watery environments.” But this raises a number of questions:
a.) If you’re hunting or foraging in a watery environment, but your hands have been immersed for fewer than 20 minutes so they’re unwrinkled, you’re better off gripping a dry object with dry hands instead of wet ones. You have an advantage with wrinkled fingers only if they’ve been underwater long enough to get wrinkled, and that advantage is only over unwrinkled wet fingers so long as you’re gripping an object that is itself wet, like a plastic disk that your fingers have wetted. If you’re trying to grab a dry object when your hands are wet and wrinkled, you’re worse off than when using dry hands.
b.) They did not test the three conditions when gripping large dry objects like a tree branch or an animal, which may not behave like plastic disks! This is essential if you think that either grabbing dry objects was important for our ancestors even when our fingers were wrinkled from having been immersed in water.
b.) We did not evolve in a watery environment; the “aquatic ape” hypothesis has long been dispelled. As for our relatives, the BBC article says “only one other primate has so far been found to have water-induced wrinkling of the fingers—Japanese macaques.” (Naturally, they show a photo of a macaque sitting in water.) I’m not sure if other primates have eve been tested (no such tests are referenced), but if chimps, bonobos, and orangs show finger wrinkling, that would imply that it did NOT evolve to enhance grip strength in watery environments. These primates don’t live in those environments!
d) What about doing the study with dry wrinkled fingers? (You quickly dry them before grasping the object.) The adaptive hypothesis would predict that there would be no grasping advantage of dry wrinkled fingers over dry unwrinkled fingers. They didn’t do that experiment (as far as I can see).
e.) What about the TOES? They get wrinkled too. The paper posit that wrinkled toes would aid “travel and clambering in wet areas”, but that is pure speculation—not even a hypothesis. It could be fairly easily tested, but wasn’t.
f.) If wrinkly skin is pretty much as good as dry skin for gripping almost anything, why don’t we have permanently wrinkled skin? Author Davis has an answer:
A previous study of object manipulation with wrinkled fingers found that wet objects were moved more quickly when the fingers were wrinkly compared to dry [15]. Importantly, there is no difference in tactile sensitivity in wrinkled fingers compared to dry [16], meaning that people are not trading off acuity for friction at the fingertip. It is therefore reasonable to wonder why healthy people do not have permanently wrinkled fingers. The answer presumably lies in the changes in the mechanical properties of the finger tissues, where there may be lower shear resistance when the finger is wrinkled [17]. Previous studies have also suggested differences in manipulation across the lifespan [18–20]; the present results show age-related effects, although they are rather weak in this sample. Our journey through life leads us to develop strategies for handling familiar and unfamiliar objects, so it seems likely that strategic changes, along with sensory and motor changes, will affect how children and adults perform tasks with handheld objects [21].
Here we have ultimate pan-selectionism: if your hypothesis fails to explain another phenomenon, you simply make up a reason why that’s also adaptive. In this case, Davis posits “lower shear resistance” for wrinkled fingers, which for a reason he fails to specify must confer a disadvantage (presumably because you can’t hold onto an object as tightly).
I’m not at all convinced by this explanation or the supporting data, as they’re contradicted by evolutionary observations and by the absence of data on wrinkled toes. As the BBC says, some believe that wrinkling “could just be a coincidental physiological response with no adaptive function.” (Go have a look at that link!). I am one of those skeptics. What surprises me is that that statement is the sing caveat (and doesn’t reprise what’s at the link) in a whole article pushing the “adaptive wrinkling in wet environments” hypothesis.
Other venues have also picked up this result, and I guess they are either overly credulous or didn’t read the paper carefully enough. Or they didn’t ask probing questions.
I can’t remember why I opened the natural-selection chapter in Why Evolution is True (chapter 5: “The Engine of Evolution”) with the story of the Asian giant hornet (Vespa mandarina) and of the counterdefense of its prey of native honeybees. (The European honeybee, more recently introduced into Asia, has not evolved such a bizarre and amazing defense.) The giant hornet is much to be feared by both honeybees and humans: it’s as big as your thumb, and several humans (and millions of bees) die from its attacks every year.
Since you all should have a copy of WEIT (as Hitch would say, “Available at fine bookstores everywhere”), I won’t recount the story of the native honeybees’ defense, but it involve luring the voracious hornet scout into the bee nest and the cooking it to death: surrounding it with a ball of vibrating bee bodies that raises the ball’s internal temperature to 117 degrees F (47°C): a temperature that kills the wasp but not the bees.
I suppose I put that story in because it’s a stunning example of the power of natural selection to shape behavior (in both wasp and bee), and not many people knew about it. Now I hear that a lot of readers especially liked that story. It is a true one, and in this segment from BBC Earth, you can see the nefarious hornet scout discovering a hive of native honeybees.
The scout marks the hive with pheromones and usually flies back to recruit a swarm of fellow hornets to return to the nest to destroy it: a process that can take only a short while as the wasps nest in minutes, decapitate adult bees and steal their honey and and bee grubs. But, as I relate in the book, sometimes, as here, the hornet scout never gets back to its own nest because of the counterdefense. The native bees lure it inside and cover it with vibrating bees that kill it.
This video is, of course, far more vivid that what I could say in words, so I want to show it here. But imagine the sequence of evolutionary steps that produced this defense!
If you want to see how these hornets slaughter the non-native European honeybees, watch this gruesome attack (each wasp can kill 40 bees per minute!). I’m sure I’ve shown this video somewhere in the distant past.
Here we have a case of selection by humans—killing elephants that have tusks because ivory is so valuable—increasing the frequency of tuskless African elephants in Mozambique over a 28-year period. (As we’ll see, only the proportion of tuskless females increased.) We have similar examples from other species, as in the reduction of horn size in bighorn sheep hunted for their horns as trophies, and the reduction in the size of some fish due to commercial fisherman going after the big ones.
Is this artificial or natural selection? Well, you could say it’s artificial selection because humans are doing the choosing, but after all human are part of nature. And this selection was not conducted to arrive at a given end. Dachshunds were selected to look like hot dogs to root out badgers in their burrows, but the reduction of tusk size in elephant, or horns in sheep, was not a deliberate target of selection, but a byproduct of greed. So I would hesitate to characterize this as artificial selection, since it’s not like breeders choosing a given characteristic to effect a desired change. In fact, the evolutionary change that occurred is the opposite of what the “selectors” wanted.
You can find the article in Science by clicking on the screenshot below, or get the pdf here. There’s a two page shorter take that’s an easier read, “Of war, tusks, and genes,” here.
The phenomenon: a civil war in Mozambique from 1977 to 1992, which increased the frequency of tuskless female elephants from 18.5% to 50.9%, nearly a threefold increase. Why? A model showed that such a change (which occurs among generations, so it’s not just selective killing within a generation) must have been due to natural selection rather than genetic drift. The killing was motivated by a desire to get money to fund the conflict. A female without tusks had five times the chance of surviving as a tusked female. That imposed strong selection in favor of tuskless females.
Usually, tuskless elephants are at a disadvantage, for tusks are multi-use features, employed for defense, digging holes for water, male-male competition, and stripping bark from trees to get food. But the natural selection to keep tusks in females was weaker than the “artificial selection” by humans against tusks.
Here’s a photo of a tuskless vs. a tusked female:
Photo by Finbarr O’Reilly for The New York Times
And the only kind of male that we see: ones with big tusks (tusk size varies, of course, as they continue to grow as the elephant lives). Tusks are homologous with our incisor teeth.
The authors first tried to determine the genetic basis of having versus lacking tusks. It turns out that, by and large, tusklessness behaves not as a complex trait caused by changes in many genes of small effect, but as a single dominant mutation on the X chromosome (like us, elephant males are XY and females are XX). Further, the dominant mutation causing tusklessness is lethal in males, killing them before birth. (This is probably not because the tuskless gene form is itself lethal, but is closely linked to a gene that is a recessive lethal.)
So here are the “genotypes” of the elephants. I’ve used “x” as the gene form on the X chromosome that produces tusks, and “X” as the alternative dominant allele that makes you tuskless.
Males: All have tusks and are thus xY. (Males have only one X chromosome and also a Y.) The XY genotype is lethal, so we never see males carrying the tuskless gene form (XY). Ergo, there are no tuskless males.
Females: We see two types:
Tuskless: Xx. These females will lose half their male offspring because when mated to an xy male (the only viable type), they produce half xY males, which are tuskers, and half XY males, which are lethal. Thus a population of tuskless females will produce a sex ratio in their offspring skewed towards females, which is what is observed.
We never see XX tuskless females because they’d have to inherit one “X” from from their fathers, but that XY genotype is lethal.
With tusks: xx.
There are a few complications, as other genes are involved (for example tusked mothers, who are xx, produce only 91% of tusked daughters when you’d expect the xx by xY cross to produce 100% xx (tusked) daughters. So things are not quite so simple, but in general a single gene seems largely responsible for the tuskless condition. (You might expect this, because if many genes were involved you simply wouldn’t get females lacking tusks: you’d get females with slightly smaller tusks, who would still be killed for their ivory. It would thus take many generations instead of a couple to raise the frequency of tuskless females.)
I won’t go into the gory genetic details, but the authors sequenced entire genomes from tusked and tuskless males and females and looked for signs of natural selection on some genes, comparing the tusked versus tuskless females. (One sign of rapid selection for tusklessness, for the cognoscenti, is the presence of DNA bases recurrent and common near the gene causing tusklessness.)
The researchers found one X-linked gene form with strong signs of selection called AMELX, which in other mammals codes for a protein that leads to the mineralization of enamel and regulates other tooth-associated genes. Another gene not on the sex chromosome, MEP1a, also is associated with tusklessness, but not as strongly. This gene, too, is known to be associated with tooth formation in other mammals. Here’s the diagram from the paper of which parts of the tusk are controlled by which gene. You can see that AMELX is expressed only in the “tusky” part of the tusk:
(From paper): Putative functional effects of candidate loci on tusk morphology.A cross section of an African elephant tusk shows the anatomical position of (a) enamel, (b) cementum, (c) dentin (ivory), (d) periodontium, and (e) root of the tusk. Dark blue circles indicate regions known or proposed to be affected by candidate gene AMELX. Light blue circles are proposed to be affected by candidate gene MEP1a. Neither gene is known to affect the formation of the dental pulp (black interior of cross section).
The upshot: Human-imposed (“anthropogenic”) selection that causes evolution in the wild has been demonstrated before, so this phenomenon is not new. What is new is that the genes involved in an anthropogenic evolutionary change—the increase in frequency of the tuskless allele, which is evolution—have been identified for the first time, and we know the kind of selection that’s caused the evolution. What is also unusual (I know of no other case) is that selection for tusklessness is in opposite directions (“antagonistic selection”) in the two sexes so long as tuskless females survive better. As the authors note:
Physical linkage between AMELX and proximate male-lethal loci on the X chromosome, such as HCCS, may underpin the proposed X-linked dominant, male-lethal inheritance of tusklessness in the Gorongosa population. If our interpretation is correct, this study represents a rare example of human-mediated selection favoring a female-specific trait despite its previously unknown deleterious effect in males (sexually antagonistic selection). Given the timeframe of selection, speed of evolutionary response, and known presence of the selected phenotype before the selective event, the selection of standing genetic variation at these loci is the most plausible explanation for the rapid rise of tusklessness during this 15-year period of conflict.
What of the future? Even though the conflict is over, poachers continue to kill tuskers for their ivory in much of Africa. What will happen? We expect the frequency of the dominant tuskless allele to increase. That itself will not lead to extinction of the population because tuskless males are simply not produced: all tuskless females will remain Xx and produce half the normal number of males. Tusked females will still be produced as Xx females crossed to xY males will produce both Xx (tuskless) and xx (tusked) females. But the reduction in the number of males produced by anthropogenic selection, coupled with continual poaching of both males and females with tusks may drive the population size so low, with an unequal sex ratio, that it could become severely endangered.
Since tusks are good for elephants, the solution is not only to ban the trade in ivory, which has been done in part, but some countries continue to trade in elephant ivory. Further, we must stop the poachers cold, as there’s still a market for both legal and illegal ivory, and prices are high. That’s easier said than done given the area that must be monitored. Note, though, that in 2017, Donald Trump lifted the ban on ivory imports from Zimbabwe, which had been put in place by his predecessor. And the elephant is the Republican symbol!
Although Darwin himself drew a bit of a distinction between natural and sexual selection, the latter is really a special case of the former. Sexual selection is simply natural selection among individuals for their ability to acquire a mate: one of many behaviors that determine how many genes you leave behind. And there are cases in which it’s hard to determine which form of selection is going on. If a male’s sperm swim faster than the sperm of other males in a species where females are multiply inseminated (e.g., fruit flies), is that male experiencing positive natural selection or positive sexual selection?
Well, the details don’t matter so long as we keep track of what’s going on. In a new paper in Nature Communications, also summarized in a short News and Views in Current Biology, a group of investigators demonstrate how sexual selection can conflict with other forms of natural selection. The experiment was hard and laborious, but the results can be conveyed simply, and I’ll try.
I’d suggest that if you read one of the two articles, it should be the second, as it’s shorter, written for a less specialized audience, but nevertheless an accurate summary. But if you want the original paper, click on the screenshot below or get the pdf here.
To read the Current Biology precis, click on the screenshot below or get the pdf here.
We begin with a sexually dimorphic beetle (below), Gnatocerus comutus, the “broad-horned flour beetle” that’s a pest in grain silos. As you see, it’s sexually dimorphic, with males having bigger heads and, notably, a huge pair of mandibles (arrows). The females lack mandibles. That’s a hint that the mandibles aren’t used for defense against predators or for predation, but are used in male-male competition for females (if they helped procure prey or fight off predators, the females should have them, too). And indeed, that’s exactly what the mandibles are used for.
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A prediction from this difference is that there is a metabolic cost to growing those mandibles, and although males with mandibles have higher overall fitness, if you could remove male-male competition, the mandibles wouldn’t give you a selective advantage. In that case they would be selected to evolve a smaller size as the resources used to grow them could be directed at other aspects of fitness. Every time you see a case of sexual dimorphism involving a cumbersome or conspicuous trait, you can predict that that trait has a cost, and is involved in sexual selection (the male peacock’s tail is the classic example).
The authors of the first paper did a clever experiment. Instead of removing male-male competition (you could do this by pairing one male with one female for generations; I predict the mandibles would get smaller), they exposed the males and females (separately) to a vicious predator, the assassin bug Amphibolus venator, which doesn’t regularly prey on G. comutus in nature but will eat anything it encounters.
Here’s the assassin bug confronting its potential prey (from the Current Biology paper):
What happened?
First, over 7 generations, with the males who escaped predation mated to control (unselected) females, the offspring of the escaping males evolved a smaller size. Clearly they weren’t defending themselves against predation from the assassin bugs; rather, the mandibles appear to have been an impediment to survival. The authors suggest that they’re heavy and impede the mobility you need to escape predators.
And, as expected, those small-jawed males whose descendants survived 7 generations of predation lost out when allowed to compete with regular males for females: they won contests only half as often as males from control treatments or female-only predation treatments. Jaws matter at mating time!
What was not expected was that the female descendants of the predated males actually got fitter. Why? Because their abdomens got larger, possibly enabling them to produce more eggs. (An alternative is that females’ sperm storage organs got larger, enabling them to store more sperm.) But why would this happen? Probably because there is a genetic correlation between male mandible size and, in females, either abdomen or sperm-storage organ size, so if you make the former smaller, the latter get bigger. There’s independent evidence for this. (We don’t know about the developmental pathways that connect male jaws and female abdomens.)
What this shows is not only the cost of sexual selection, but a cost that’s levied in both males and females. If there were no male-male competition, and males had small mandibles, females would leave more offspring. You might ask, then, given that there are of evolving mandibles paid by both sexes, why do males still evolve large jaws?
The answer must be that the genes that increased male mandible size in the past still had a NET advantage over genes for smaller mandibles. In other words, their cost in reduced ability to escape predators and reduced female offspring number was more than offset by the advantage of winning contests for females. This shows that fitness increases in one sex (the larger mandibles that evolved in males) can be paid for by fitness reductions in the other sex as well (reduced reproductive output of smaller-bellied females).
And so Nature has woven a tangled web here, but one somewhat untangled by the tedious but revealing experiments of the researchers who wrote the first paper.
A big group of researchers from around the world—science is truly international in this case—just published a paper in Proceedings of the National Academy of Sciences that involved sequencing the complete genome of 18 species of penguins as well as an outgroup, the southern giant petrel. (Researchers differ on the number of extant penguin species, ranging from 17 to 20, as some populations are geographically isolated, making it hard to discern species status.)
The DNA information was combined with fossil data to yield a family tree of the living species, and also to reconstruct their evolutionary history, which suggested that the ancestor of all living and fossil penguins probably lived not in Antarctica, but on the coasts of Australia and/or New Zealand. Finally, the researchers were able to narrow in on a group of genes that may have undergone natural selection in the group, suggesting which adaptations were crucial for making a well-functioning penguin.
You can access the paper by clicking on the screenshot below, or see the pdf here. The full reference is at the bottom, and there’s a popular summary article at CNN.
I’ll try to be brief here. First, I’ve put below the family tree of living penguins deduced from the DNA information, with the divergence times that come from both DNA and fossil data. The radiation started around the beginning of the Miocene, roughly 22 million years ago.
As you can see, the largest species—the emperor and king penguins, form their own “outgroup” to the rest of the penguins, splitting off from the rest early in the group’s radiation but splitting from each other only about two million years ago. (Despite the radiation being old, most modern species split from their closest relatives only within the last few million years.)
The average temperature of the southern ocean is given by the graph in white and the scale on the left, with the dotted red line showing the beginning of the “strengthening” of the Antarctic Circumpolar Current (ACC), a strong ocean current that sweeps clockwise around Antarctica as seen from the South Pole, isolating the continent from warmer ocean temperatures to the north and allowing the ice sheet to persist. A lot of the radiation followed the advent of this current’s new strength, which also coincided with the opening of the Drake Passage, creating a water gap between the previously connected land masses of Antarctica and South America. It also produced a lot of sub-Antarctic islands that were also sites for colonization. And geographic isolation, possibly enforced by temperature, is an impetus for the formation of new species.
It was this stronger current and geographic separation that, the authors say, prompted new speciation events in penguins (most biologists assume that new species usually arise after populations become geographically separated). They did, however, detect some gene flow between penguin species, though it wasn’t extensive enough to wipe out the differences that produced this tree:
Using some assumptions and a complicated program, the authors could use the phylogeny to estimate the geographic range of the ancestral species as well as the ranges of ancestors within the phylogeny. Those are indicated with the letters A through I in the figure above.
The procedure is complicated, but it’s done the way evolutionists estimate ancestral traits of species—assuming that ancestors pass traits down to their descendants. In this case “geographic range” is considered a trait of a species. For example, if two closely related but distinct species occupy geographic areas that are close together, one can assume that their joint ancestor lived in that general area as well. The figure below shows the geographic areas that correspond the the letters of existing penguins (under their names) as well as the ancestors of groups (letters at the branch points).
The range of the ancestral node is letter I, and you can see that corresponds to the coastal areas of Australia and New Zealand, which, the authors assume, is where the ancestral species that gave rise to all modern penguins lived. This is a big conclusion of the paper, but since there are numerous assumptions that go into the biogeographic model, and not a lot of fossil data, I would take that conclusion as very tentative. If it’s true, that means that penguins evolved in areas where the water temperature at the time was abut 9ºC (48° F), and then some descendants (e.g. kings and adelies) colonized colder waters, while others (e.g.. Galápagos and African penguins) colonized warmer waters.
The ancestor of king and emperor penguins presumably lived on the coast of South America or Antarctica (letters A and C); kings currently breed on subantarctic islands and emperors only in Antarctica.
It’s possible, looking at the amount of genetic variation within whole genomes, to discern something about the demographic history (i.e., population sizes) of penguin species (again, there are some big assumptions here). You see below the plot of the “effective population size” (a figure that’s usually somewhat lower than the actual census size) for six species of penguins. Most show a strong drop in population size between about 70,000 and 40,000 years ago, which corresponds to the last glacial maximum (LGM, indicated by the vertical line). The authors say that the extreme cold during the LGM may have reduced the productivity of marine waters, and hence the abundance of fish and krill, the main diet of penguins. That, in turn, is said to have reduced the population size of many penguin species:
Finally, there are ways to detect genes in a lineage that may have been subject to natural selection. This is done by finding genes in which there is an elevated rate of amino acid substitutions, which change the structure of a protein, over the rate of presumed “neutral” changes in DNA, which don’t change protein structure. The assumption here, which is a good one, is that a relatively faster rate of protein evolution was promoted by natural selection.
Here’s a diagram of some of the genes, and classes of genes, that, says the analysis, underwent (positive) natural selection, presumably conferring adaptation on individuals in the various species. The genes that apparently evolved adaptively are in pathways influencing thermoregulation, osmoregulation via renal function (fluid and salt balance), blood pressure regulation (helps conserve oxygen and maintain core body temperatures), and oxygenation (important in deep diving). Some of the genes are named in the diagram below. Again, these genes are identified as candidates for adaptation only from their pattern of DNA substitution in the tree, and we don’t know for sure whether the changes really were adaptive, much less how they affected the animal.
The authors conclude on a sad note, saying that it took penguins millions of years to adapt to new temperatures (including colonizing the relatively warm waters around the Galápagos Islands), and thus would likely be unable to adapt to the relatively fast temperature increases accompanying global warming. While one would think that a history of slow adaptation doesn’t say anything about how fast adaptation could proceed under more rapid environmental change, we already know that global warming is seriously damaging some populations of penguins. The CNN report quotes the first author of the paper and describes some heartbreaking changes:
“Right now, changes in the climate and environment are going too fast for some species to respond to the climate change,” said Juliana Vianna, associate professor at the Pontifical Catholic University of Chile, in the UC Berkeley statement.
The different elements of climate change culminate in a perfect storm. Disappearing sea ice mean fewer breeding and resting grounds for emperor penguins. The reduced ice and warming oceans also mean less krill, the main component of the penguins’ diet.
The world’s second-largest emperor penguin colony has almost disappeared; thousands of emperor penguin chicks in Antarctica drowned when sea ice was destroyed by storms in 2016. Reoccuring storms in 2017 and 2018 led to the death of almost all the chicks at the site each season.
Some penguin colonies in the Antarctic have declined by more than 75% over the past 50 years, largely as a result of climate change.
In the Galapagos, penguin populations are declining as warm El Nino events — a weather phenomenon that sees warming of the eastern Pacific Ocean — happen more frequently and with greater severity. In Africa, warming waters off the southern coast have also caused penguin populations to drop drastically.
I’m lucky to have seen five species of penguins, including kings, on my trip to Antarctica last winter. It would break my heart if we humans, through our depredation of the environment, drove these magnificent products of evolution to extinction. They were here long before we were!
h/t: Matthew, Terrance
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Vianna, J. A., F. A. N. Fernandes, M. J. Frugone, H. V. Figueiró, L. R. Pertierra, D. Noll, K. Bi, C. Y. Wang-Claypool, A. Lowther, P. Parker, C. Le Bohec, F. Bonadonna, B. Wienecke, P. Pistorius, A. Steinfurth, C. P. Burridge, G. P. M. Dantas, E. Poulin, W. B. Simison, J. Henderson, E. Eizirik, M. F. Nery, and R. C. K. Bowie. 2020. Genome-wide analyses reveal drivers of penguin diversification. Proceedings of the National Academy of Sciences:202006659.
I don’t remember encountering this case of mimicry, but it’s so amazing that, when I became aware of it from a tweet (yes, Twitter has its uses), I decided to give it a post of its own.
First the tweet, sent to me by Matthew. He added, “This is the Iranian viper, as featured in Seven Worlds, One Planet, made by the BBC. Amazing.”
Parece una araña dando vueltas sin sentido, pero es una serpiente Pseudocerastes urarachnoides moviendo su cola como señuelo para atraer a los pájaros que forman parte de su dieta. Si queréis verla en acción, aquí podéis ver una captura: https://t.co/vRhh0JJlza. #naturalezapic.twitter.com/7wJ0LjxPeV
You don’t need to translate the Spanish, though, as the video below tells all. I swear that when I first watched it, I thought there was a real spider crawling on the snake’s back.
The snake is the spider-tailed horned viper, Pseudocerastes urarachnoides, which has a small range in Western Iran (map from Wikipedia):
It wasn’t described as a new species until 2006 in the paper below (free access); before that it was thought to be the already-describe Persian horned viper. (I guess they overlooked the tail ornament.)
Here’s a photo of the tail “spider” from the paper; the one below that is from Wikipedia. The resemblance may not be precise, but (as you see above), when the ornament is moved about, it looks remarkably like a spider—certainly good enough to fool birds.
In that paper, the authors didn’t know how the tail ornament was used, but were impressed at its spider-like appearance. And they guessed accurately:
This raises the question of the elaborate and sophisticated appearance of the caudal appendage in our new species, as the waving or wriggling motion of a distinctively colored tail tip seems perfectly adequate to attract lizard and anuran prey. We can only speculate that in the case of the present species, the caudal lure serves to deceive a more specific kind of prey, such as shrews or birds. Indeed, ZMGU 1300 [the specimen number] contains an undigested, unidentified passerine bird in the stomach (the feet protruding through the body wall).
Only later, using live captive specimens, did researchers see that the ornament did indeed attract birds that the snake caught and consumed, as in the video above.
Any biologist who sees this is immediately impressed by the ability of natural selection to mold not only morphology, but the behavior of the snake: the twitching of its tail so that the spider ornament appears to “walk.” But any adaptation like this ornament must have incipient stages, and each subsequent modification must improve the adaptation—that is, it much give the snake possessing the “improved” improvement a reproductive advantage. (That advantage would derive from the better nutrition of a snake who caught more birds, and thus might have more offspring, increasing the proportion of genes for more spider-like ornaments.)
My own guess was that the ornament started with the simple twitching of the tail of an immobile snake, a twitching that might attract predators and, moreover, is already known in several snakes. After that, any mutation that modified the tail, making it look more like a spider, would give the snake a further reproductive advantage. And so we get the spider ornament, which might of course still be evolving. Concurrent with the evolution of the ornament itself would be the evolution of the snake’s tail-twitching behavior, which makes the caudal appendage resemble a spider nearly perfectly.
It turns out, of course, that I’m not the first person to think of this scenario. Discover Magazine wrote about this snake last spring, and speculated about its evolution:
“The evolution of luring is more complex than contrasting color or simple shaking — the movement is precisely adapted to duplicate prey movement frequencies, amplitudes and directions, at least in specialized cases.”It’s not uncommon for many snakes to do something similar with their tails to deceive prey. The common death adder of Australia buries itself in leaves, then writhes its tail like a worm to catch lizards and frogs. The Saharan sand viper conceals itself in sand with only its eyes and nostrils visible. When a lizard comes along, it sticks its tail out from the dirt, making it squirm like an insect larvae. The behavior — and the elaborate body modifications that can accompany it — likely arose from a behavior common to many reptiles, Schwenk explains. When they are about to strike prey, any lizards and snakes enter a hyper-alert pose. The reptiles will focus their vision by cocking their heads to the side, arching their backs, and certain species will commonly vibrate their tail tip against the ground. This can distract the prey, which will shift its attention to the vibrating tail, ignoring the reptile mouth opening to grab them.“This simple pattern leads to selection causing refining of the tail form and motion to be more attractive to such prey by more accurately mimicking actual prey movements,” Schwenk theorizes. “The other ancestral condition that could have led to caudal luring, or possibly an intermediate step in the process, is the use of tail vibration for prey distraction rather than for luring.” Indeed, those most famous tail shakers, the rattlesnakes, sometimes also use caudal luring. For example, juvenile dusky pygmy rattlesnakes, whose rattle is so small it barely makes noise, wiggle their tails to attract prey. The behavior, in fact, may be key to how rattlesnakes evolved their distinctive rears, although this theory is somewhat controversial. “Like many other apparently simple things in biology, there is a lot of complexity to caudal luring that has barely been explored,” Schwenk says. “Much of this has been considered in a piecemeal fashion, but a thorough review and synthesis … has not been attempted.”
Now we’re not sure if this is the correct evolutionary pathway, but constructing a plausible step-by-step scenario like this, and showing that the intermediate “stages” occur as adaptations among existing species, is sufficient to refute the creationist claim that structures like the spider ornament could not have evolved and thus much have been created by God (or a “designer”, which means the same thing). The same kind of argument was used by Darwin in The Origin to refute Paley’s argument that the camera eye must have been created by God. Dawkins discusses it in the video below (and, as I recall, in his book The Blind Watchmaker).
Here are two questions to ponder while I am doing other things today. The first comes from Matthew, whose words are indented:
Here’s a question which might be good to pose to readers.
Why are there no live-bearing birds? Live-birth has evolved many times in squamates, so is clearly within mutational reach of the reptilian genome (and interestingly, it generally leads to social behaviour). It has been argued that birds lay eggs because they would be too heavy to fly if they were carrying around young inside them. Apart from the obvious problem that bats manage fine, if that argument is right, you might expect some flightless birds to have been live-bearing. But they aren’t. Maybe they were in the past? Any hand-wavy explanations?
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And I have my own question:
Why are there no herbivorous snakes? There are lots of snakes in the world and they slither in the grass, but none of them eat it—or any other vegetation. They are all carnivores.
This is particuarly puzzling in light of the fact that the relatives of snakes—lizards—often eat a great deal of vegetation, and at least one species—the marine iguana of the Galápagos—eats only vegetation (algae; though rarely they’ll eat other stuff). So it is possible for reptiles to evolve into herbivores. (Many of the dinosaurs were plant-eaters.) Why haven’t snakes done it?
Neither Matthew and I know the answers here (after all, these questions bear on mutational possibility, evolutionary history, physiology, and so on), but the questions are interesting to ponder. They do show that not all conceivable “niches” get filled by evolution.
Here’s a nice video of a marine iguana (Amblyrhynchus cristatus) foraging; I saw many of these when I visited the Galápagos some years ago. It is also the only marine lizard. There are other marine reptiles like saltwater crocodiles, sea snakes, and of course marine turtles, but to my knowledge this is the only lizard that forages in the sea (they live mostly ashore).
UPDATE: I found out that the well-known evolutionary geneticist John C. Avise published a related book in 2010, but one that concentrates on a different line of evidence for evolution. John’s book (screenshot of cover below with link to Amazon) lays out the many suboptimal features of the human genome. He thus concentrates on molecular evidence, noting the many features in that bailiwick whose imperfection gives evidence for evolution and against intelligent design. Lents’s and Avise’s books thus make a good pair, since the former seems to deal mostly with anatomy and physiology and the latter with molecular data. I’ll be reading both of them.
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Biologist Nathan Lents, whose abbreviated c.v. is given below, has been featured on this site before, both as a critic of creationism (good), but also as a defender of the Adam-and-Eve apologetics pushed by his religious friend Josh Swamidass (bad). But chalk up another two marks on Lents’s “good” side. First, he’s written a book (click on screenshot below) that lays out all the suboptimal features of the human body—features whose imperfection gives evidence for evolution. I’m getting the book for teaching purposes, and here’s the Amazon summary:
Dating back to Darwin himself, the “argument from poor design” holds that examples of suboptimal structure/function demonstrate that nature does not have a designer. Perhaps surprisingly, human beings have more than our share of quirks and glitches. Besides speaking to our shared ancestry, these evolutionary “seams” reveal interesting things about our past. This offers a unique accounting of our evolutionary legacy and sheds new light on how to live in better harmony with our bodies, in all their flawed glory.
Nathan Lents is Professor of Biology at John Jay College and author of two recent books: Not So Different and Human Errors. With degrees in molecular biology and human physiology, and a postdoctoral fellowship in computational genomics, Lents tackles the evolution of human biology from a broad and interdisciplinary perspective. In addition to his research and teaching, he can be found defending sound evolutionary science in the pages of Science, Skeptic Magazine, the Wall Street Journal, The Guardian, and others.
And here’s a half-hour Center for Inquiry talk, clearly based on his book, in which Lents discusses how the flaws in the human body instantiate evolution. It’s not just that there are flaws—which support the notion that natural selection doesn’t produce absolute perfection, but simply the best result available given the existing genetic variation—but, more important: those flaws are understandable as the result of our evolution from ancestors who were different from us.
Some of Lents’s examples (like our broken gene in the Vitamin C synthesis pathway), are discussed in WEIT, but others, like the bizarre configuration of our nasal sinuses, aren’t. I haven’t seen the book, but it looks like a good compendium of evidence for evolution using something that everyone’s familiar with: the glitches and bugs in the human body.
It’s a good talk, and Lents is an energetic and lucid lecturer. I recommend that you listen to this, for you’ll learn stuff that will stay with you, and also serve to help you argue with creationists.
